As an application of a pouch type lithium ion secondary battery has spread in recent years, energy density of batteries and energy storage capacity of a unit cell have gradually increased. Particularly, to meet the requirements brought from an increase in a vehicle mileage, it has been actively studied in recent years to find a technology that can increase the energy storage capacity of a lithium ion secondary battery for vehicles. However, according to the conventional pouch type lithium ion secondary battery, a sheath of the battery has a low physical strength, and the battery is not provided with a cell level safety device (CID, PTC, fuse, etc.), and thus, it is difficult to realize safety of a battery having a high energy storage capacity. Here, the technical term CID is an abbreviation of a current interruptive device and is used as an overcharge safety device. Particularly, when a vehicle battery is overcharged, gas is generated in the battery, and the temperature inside the battery increases, thereby thermal runaway may be induced in a material inside the battery which may result in a fire. Further, when an abnormal reaction is generated in even just one unit cell during an operation of a vehicle, an abnormal voltage may be easily induced in the battery pack, thereby reducing the driving safety of the vehicle.
The foregoing is intended merely to aid in the understanding of the background of the present disclosure, and is not intended to mean that the present disclosure falls within the purview of the related art that is already known to those skilled in the art.